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EDITORIAL |
China
Endovascular aortic stent grafting remained one of the most significant developments in vascular surgery in the last decade since its first introduction in 1991. Today at least a dozen commercial devices are available for treating a wide spectrum of diseases in the thoracic and abdominal aorta. The minimally invasive concept of transfemoral introduction of a self-expanding stent graft system using a small groin incision, or even percutaneously, often under local or minimal anaesthesia, is appealing to surgeons and patients alike. Considerable experience has been gained from endovascular repair of abdominal aortic aneurysms, and we are seeing an increase in the usage of this technology to treat more complex abdominal and thoracic diseases.
ENDOVASCULAR REPAIR FOR ABDOMINAL AORTIC ANEURYSM (AAA)
Compared to the gold standard of open aneurysm repair, endovascular repair for AAA (EVAR) has the advantage of avoiding laparotomy, aortic clamping, and minimizing blood loss, resulting in shorter intensive care and hospital stays. Large prospective randomized studies such as the UK EVAR-11 and DREAM2 trials have confirmed that EVAR has a significantly lower hospital mortality (1.7 vs 4.7%) and reduced morbidity compared to traditional open repair. The price is a higher incidence of access related injuries, and a need for continuous surveillance with a possibility of secondary interventions and rupture.
The Achilles heel of EVAR is its durability. Critiques state that the hospital stay and quality of life advantages will disappear with the tedious follow up protocol. The procedure is more expensive, and the results are poorer in larger aneurysms with complex anatomy. Indeed earlier registries such as the EUROSTAR reported re-intervention and rupture rates as high as 5% and 1% per year respectively, mainly for endoleaks and migration3. As a result ingenious radiological or laparoscopic methods to treat these issues have been developed, and miniature sensors have been used to monitor intrasac pressure to detect endotension. The recently published mid term results of the EVAR-1 trial indicated there is only a 3% advantage of aneurysm-related mortality of EVAR compared with open aneurysm repair, at the expense of EVAR being 33% more expensive, has greater number of complications and re-interventions, and no difference in quality of life and overall survival at four years.4 Nevertheless due to the advantage in immediate mortality and minimal invasive nature EVAR has remained a very viable alternative for many patients, and often the preferred treatment due to the perceived lower operative risks.
In the last few years we witnessed a substantial technology improvement as newer third generation devices with added fixation and smaller profiles were introduced and better long term results are being achieved. Device specific trial data report two-year rupture rates of less than 1%, migration 2–3%, endoleaks 10–15%, and conversion 1–3%. There is continuous debate as to whether EVAR should be used to treat high-risk patients compared to no treatment. The EVAR-2 Trial showed that endovascular repair in patients at high surgical risk is not worthwhile, due to an initial high operative mortality5. On the other hand, the results of EVAR are most favorable in small aneurysms. With the recommendation by the US Preventive Services Task Force on one time screening for AAA in men aged 65–75 who had smoked, it is likely that the issue of early treatment of small AAA will surface. Two trials randomizing small AAAs to EVAR and conservative treatment, the PIVOTAL trial in the USA and the CAESAR trial in Europe, will provide the answer to whether small AAAs could be effectively treated by early EVAR.
It has to be kept in mind that regulation trial results may not reflect real life practice, where selection guidelines are often compromised and the applicability of EVAR continues to evolve. There has been an increasing interest to use EVAR in treating ruptured aneurysms. Fenestrated stent grafts and branched stent grafts have been developed to treat aneurysms with complicated anatomy and short necks. At this time EVAR is technically safe, with lower morbidity and mortality than open surgery. The need for extended follow up remains, and the long term success rate is not yet known.
ENDOVASCULAR STENT GRAFT FOR THORACIC PATHOLOGY
The use of endovascular stent grafts to treat thoracic diseases was particularly attractive since it avoids the morbidity from thoracotomy and extracorporeal circulation. The main indications are thoracic aortic aneurysms and dissections, while other applications include aortic penetrating ulcers, traumatic rupture, mycotic aneurysms and even aorto-esophageal fistulas.6
The introduction of thoracic endografts has lagged behind EVAR for AAA. Current delivery systems are of larger caliber (22–25Fr) and pose access issues particularly in Asian women with smaller iliac arteries. Not uncommonly an iliac conduit has to be constructed for access. As the pathology approaches the aortic arch, additional difficulties arise from diameter differences, aortic tortuosity, graft movement with aortic pulsation, and the "windsock" effect from antegrade blood flow, rendering accurate deployment difficult. The force of blood flow and curvature in the descending aorta tend to distract the thoracic stent graft leading to proximal migration, and at least one device with distal fixation has been marketed. Other problems include wire breakage and stent erosion, and difficulties in negotiating the arch. Newer grafts of smaller caliber, more flexible and trackable delivery systems have partially resolved these latter issues.
The major challenge of thoracic stent grafting is the anatomy of the arch and its branches in relation to the pathology. At least 2cm of proximal and distal landing zones are required for secure fixation. Although prevailing evidence suggests that the left subclavian artery origin may be covered with little consequence, the risk of a posterior circulation stroke remain real in patients with incomplete circles of Willis or atrophic right vertebral systems. If additional proximal coverage is needed, a carotid-left subclavian, carotid-carotid, or ascending aorta to carotids bypass may be necessary. Currently several homemade branched devices to preserve the major arch vessels have been developed in Japan and China, but the deployment process is complicated, and their utilization limited. For extensive diseases, covering the majority of the intercostal arteries will pose a danger of paraplegia. In the Gore Phase II Multicenter trial, a paraplegia rate of 3% has been reported.7
An emerging indication for thoracic stent graft is in the treatment of aortic dissections. Complicated aortic dissection requiring surgery has a mortality rate in excess of 25%, and long term survival of these patient are reduced due to late complications from aneurysmal dilatation and rupture. The use of endograft to treat aortic dissection was pioneered by the Stanford group, with the aim of covering the primary tear and promoting false lumen thrombosis. Additional secondary procedures may include the use of distal bare aortic stents to assist in re-expansion of the true lumen, covered extension pieces to seal secondary fenestrations, and stenting of the visceral branch vessels. Early results are promising, although controversies still exist as to the indications, suitability for Type A or Type B disease, optimal timing of the procedure, and whether one should treat uncomplicated, asymptomatic chronic dissections. There has been a wide variation of practice, and long term results are lacking. A randomized trial (INSTEAD) is currently in progress to study the benefits of endovascular repair of subacute and chronic Type B dissections.
On the horizon is the usage of stent graft technology in combination with traditional open surgery in various hybrid procedures. Proximal aortic replacement can be combined with an elephant trunk procedure to facilitate staged placement of a distal thoracic endograft. Abdominal visceral transpositions and bypasses can be performed to enable concomitant endovascular repair of more complex thoracoabdominal aneurysms. With innovation of the surgeon and improvement in technology, we will witness the continuing evolvement of endovascular stent grafting in treating more diseases of the aorta. The ultimate formula for success remained a dedicated team of operators with good facilities and training, and meticulous planning taking into consideration of arterial anatomy, patient risk profile and life expectancy.
REFERENCES
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